A typical overhead door assembly comprises a door composed of panels connected together by horizontal hinges. The individual door panels may roll on tracks at the opposite sides of the door opening, there being suitable small roller carriages interconnecting the panels and the tracks. The door and tracks are arranged so that when the door is closed it fits tightly around the door opening.
The door assembly also includes a mechanism to facilitate opening and closing the door. Usually this mechanism employs a horizontal shaft which is rotatably mounted above the door opening. Pulleys are present adjacent to the opposite ends of the shaft around which are wound cables. Corresponding first ends of these cables are connected to the corresponding pulleys. The opposite ends of those cables are secured to opposite sides of the lowest panel of the door.
Such door assemblies also invariably include means for counterbalancing the weight of the door. In many popular overhead door assemblies, the counterbalancing means comprise one or more springs coiled around the shaft. One end of each spring is fixed, while the other end of each spring is secured to the shaft for rotation therewith. The spring(s) impart a torque to the shaft and pulleys thereon which tension the cables sufficiently to almost offset the weight of the door so that the door can be opened and closed easily by a small upward or downward pulling force on the door. Usually, a short strap or rope is provided at the side of the lowest door panel to facilitate opening and closing the door.
In all overhead door installations, the spring bias provided by the counterbalancing spring(s) is set during installation by adjusting one end of each spring so that the total spring bias just offsets the weight of the original door. In such cases, it has been found that the counterbalance becomes upset over time so that more or less force is required to open or close the door. This upset may be due to aging of the counterbalancing spring(s) or to added door weight, such as when the door is painted, etc. As a result, over time, the door may not close completely or may slam closed.
Sometimes people are tempted to adjust the counterbalancing mechanism so that the door will close correctly. This raises the possibility that an inexperienced, unauthorized and/or non-technical person could become injured in the process of making the tension adjustment. In other words, when fully wound up or loaded, the counterbalancing spring(s), each of which may be several feet long, possess considerable potential energy. Therefore, if one end of the counterbalancing spring(s) should be released suddenly, the spring(s) would instantly unwind causing possible injury to an unknowledgeable homeowner or other servicer.